1. Field of the Invention
This invention relates to an improvement of a lead chalcogenide semiconductor device, particularly to a structure of novel P conductivity type region contact for lead-salt diode lasers operating in the infrared region.
2. Description of the Prior Art
In general, the lead chalcogenide semiconductors such as lead, sulfur and selenium (Pb, S, Se) are often used as the materials for electro-optical conversion elements in the infrared region. In such a semiconductor element, it is very important to provide an excellent ohmic contact electrode having a low resistance.
A semiconductor light emitting element using a lead chalcogenide multi-element semiconductor as the materials thereof may be a semiconductor laser element (hereinafter referred to only as a laser element). If this is the case, if the electrode has a high contact resistance or is a non-ohmic contact electrode, the wavelength of laser beam fluctuates or it becomes difficult, in the worst case, to allow said laser element to continuously oscillate because the Joule heat is generated at the said electrode part due to a drive current flowing through said laser element and an electrode resistance. This results in an increase in the temperature of the element.
An element structure of an ordinary semiconductor laser element which operates in the infrared region may include a heat sink. The semiconductor part of laser element consists of PbSSe and has n and p conductivity type regions. The laser beam is emitted in a direction from the pn junction surface. The mirror surfaces which structure the resonator are formed as in the case of ordinary laser elements at the element surface from which the laser beam is emitted and its opposite surface.
In addition, the contact electrodes are provided at the n and p conductivity type semiconductor area surfaces, respectively.
In the laser element having such structure, the one contact electrode is provided by the so-called chip bonding method on the copper block coated with indium (In) and functioning as the heat sink. A gold ribbon is bonded via indium brazing on the other contact electrode. The copper block and the gold ribbon thus function as lead-out electrodes.
As explained, gold (Au) has been used ordinarily as the material of the contact electrodes of a laser element consisting of the PbSSe. In this case, since the satisfactory low resistance ohmic contact can be obtained at the surface of the n conductivity type region of the PbSSe semiconductor, because the functions of Au and contact area are almost equal, no particular problem occurs.
However, the p conductivity type region of the PbSSe semiconductor has the disadvantage of causing Joule heat because such region has a comparatively high resistance non-ohmic contact characteristic due to the Schottky barrier generated in a contact area between the p conductivity type semiconductor region and the Au contact electrode. For this reason, ultra-low temperature cooling technology utilizing liquid helium (He) is indispensable for realizing continuous light emitting operation of the lead chalcogenide semiconductor laser element of this type. This requirement has thus made the cost of manufacture and the cost of operation of the device very expensive.
Moreover, a problem of the p conductivity type region contact resistance is also reported in a paper entitled, "Contact Reliability Studies on Lead-Salt Diode Lasers" by W. Lo et al, "Journal of Electrochemical Society", Vol. 127, No. 6, pp. 1372 to 1375. This paper proposes a double-layered Au-Pt electrode structure in order to prevent slow increase in contact resistance due to the migration of indium (In). However, the structure proposed above is still insufficient for decreasing the contact resistance value of the p conductivity type region and thereby eliminating the influence of generation of Joule heat.
It is generally thought advantageous to use a p.sup.+ conductivity type electrode contact area of the p conductivity type region in order to obtain the low resistance ohmic contact characteristic of the electrode which is in contact with the surface of said p conductivity type region of said multi-element semiconductor PbSSe. In this case, however, it is currently difficult to previously form the p.sup.+ conductivity type layer on the p conductivity type surface of said semiconductor. In other words, the liquid phase epitaxial growth method which is usually employed for formation of such p.sup.+ conductivity type layer is not yet established in the field of lead chalcogenide semiconductors including S and Se of this type. Furthermore, it is still impossible to form the p.sup.+ conductivity type layer on the surface with satisfactory reproducibility by thermal diffusion method, because control of depth is still difficult.
The principle object of the invention is to provide a lead chalcogenide semiconductor device having a decreased contact resistance.
An object of the invention is to provide a lead chalcogenide multi-element semiconductor device having an electrode structure which assures satisfactory ohmic contact without generation of Schottky barrier on the surface of the p conductivity type region of the lead-chalcogenide multi-element semiconductor device.
Another object of the invention is to provide a ternary lead salt semiconductor diode laser in which the influence of heat generation due to contact resistance is eliminated and stable and continuous oscillation operation is assured.